Storyline 360 – How Do You Add Subtle Background Music Without It Overpowering Narration?

Here's the User Guide article about the background audio that Phil mentioned: https://www.articulatesupport.com/article/Storyline-360-Background-Audio

Personally, I recommend against background music. It can be distracting, especially to someone who doesn't like the chosen music. Unless it's providing content (for example, in a course about music), just having it as extra audio increases cognitive load.